Absorption spectroscopy refers to a spectroscopic approach used to identify the presence of a substance within a sample and measure its quantity. The characterization of this technique is based on its properties, which determine the absorption of light by a sample. These properties correspond to the energy difference between the electronic states of the molecules and can be described in terms of either their frequency or wavelength.

In absorption spectroscopy, the sample interacts with the light of different frequencies or wavelengths, and the amount of light absorbed by the sample at each frequency is measured. The resulting absorption spectrum shows the amount of light absorbed by the sample at each frequency, and this absorption pattern is unique to the substance present in the sample. Therefore, the absorption spectrum is a graphical representation of the amount of light absorbed by the sample at different frequencies, and it provides valuable information about the identity and concentration of the substance in the sample.

Figure 1: Absorption Spectroscopy

This method has applications in various analytical contexts, with near-infrared (NIR) and ultraviolet-visible (UV-VIS) spectroscopy. The absorption spectroscopy technique is shown in figure 1.

Absorption Spectrum

Figure 2: Absorption Spectrum

An absorption spectrum is a graphical representation of the wavelengths of electromagnetic radiation absorbed by a substance, usually in the form of a plot of absorbance or transmittance against wavelength or frequency. The dark bands of the spectrum represent the frequencies of light that are not transmitted which the sample absorbs. These bands appear when electrons absorb energy, causing them to move from their ground state to higher energy states. This type of spectrum arises from the absorption of energy by atoms. Figure 2 shows the representation of an absorption spectrum.

When a light beam from a source pass through a solution or vapor, a pattern consisting of dark lines is generated. This pattern is examined using a spectroscope. The chemical or element present in the sample selectively absorbs certain radiations as the light passes through it, and these absorptions depend upon the nature of the chemical or element.

The dark lines in the absorption spectrum correspond precisely to the positions where colored lines are visible in the emission spectrum. This is because the energy transitions involved in absorption and emission are the reverse of each other. In other words, the energy that is absorbed during excitation is released during emission, and this release of energy occurs at the same wavelengths as the absorption. Therefore, the resulting spectrum is called the absorption spectrum.

While emission spectra can exhibit all the colors in the electromagnetic spectrum, absorption spectra might not show a few colors because of the absorption and redirection of photons. By examining the wavelengths of light that are absorbed, it is possible to determine the number of substances present in the sample.

Principle of Absorption Spectroscopy

The principle behind absorption spectroscopy is based on atoms and ions, where atoms are the fundamental units of matter and the building blocks of chemical elements while ions are charged atoms or molecules. Free atoms can absorb and emit light at specific wavelengths and absorption spectra defines the amount of energy required to excite them to higher energy states. This spectrum allows for the identification of the chemical composition of a solution by analyzing the absorption patterns.

A device known as a spectrometer is used to detect changes in the wavelengths of electromagnetic radiation that are either absorbed or emitted by a sample. The absorption spectra captured by a spectrometer are specific to different materials, which makes it a valuable tool in many fields. A spectrophotometer measures the intensity of these wavelengths and uses a light source and a monochromator to generate a beam within a specific region of the electromagnetic spectrum that can be directed towards the sample. Detectors are also used to analyze the intensity of the signal, which is then displayed on a screen for the user's analysis.

Types of Absorption Spectroscopy

The different types of absorption spectroscopy are:

• X-ray absorption spectroscopy (XAS): It uses X-rays to determine the atomic structure, electronic state, and magnetic properties of a sample.
• Ultraviolet-visible absorption spectroscopy: It measures the absorption of light wavelengths within the ultraviolet and visible regions of the electromagnetic spectrum.
• Infrared absorption spectroscopy: It is used to measure the absorption spectra of samples within the infrared region of the electromagnetic spectrum.
• Microwave absorption spectroscopy: It uses microwave radiation to induce changes in the rotational energy of molecules, producing unique absorption patterns that can be used to characterize them.

Applications of Absorption Spectroscopy

Absorption spectroscopy is widely used in medical diagnostics. Infrared (IR) absorption spectroscopy is used in the medical field to analyze the chemical composition of various samples, including blood, urine, and tissue. This technique is particularly useful in diagnosing diseases such as cancer, which often has distinct IR spectra.

Ultraviolet-visible (UV-Vis) absorption spectroscopy is used in environmental science to measure the concentration of pollutants in water and air samples. It can also monitor the effects of air pollution on plants and animals.

In the field of materials science, absorption spectroscopy is used to investigate the properties of materials such as polymers, ceramics, and metals. X-ray absorption spectroscopy can provide information on the electronic structure and bonding of materials at the atomic level. XAS is also used in the study of catalysts, which are important in many industrial processes.

It is used in astronomy to determine chemical composition, temperature, and other properties by analyzing the absorption spectra of light emitted by stars, planets, and other celestial objects. It is also used in remote sensing to detect the presence of hazardous elements without requiring physical contact with the measuring device.

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